I suggest the 7500 instead of the 7000. Good choice. I have same configuration in a machine.
I know the guy who owns dvnation. He's cool but he's also cautious. You'll be fine. Tiger works just fine.
You do not need Leopard to use Mtron or any other SSD.
Tiger is actually a better choice for reasons listed in other threads on this forum. Either one will be just fine though.
If you have Leopard and need to use the Mac Pro for things other than just Catalyst, consider upgrading. Otherwise you don't need it.
The x1900XT is the best card for Catalyst. You don't need the SCSI card anymore. The SCSI is not necessary if you have an Mtron SSD.
You say "public IP." Catalyst uses a multicast format which can overload a busy network. If you plan on using IP for Catalyst I suggest you only use Catalyst on a small network with only the devices you need. If you need to access Catalyst on a different network, use a router to pass network data from public network to the private Catalyst network.
If you need help with this, you can contact me off board. I have consulted several systems such as this before.